-Notice: There is no universally accepted method for choosing , but the mentioned approach is what mostly agreed. Also, in the second point, we can conclude our review without doing meta-analysis, specifically if there is high heterogeneity.

-Fixed effect analysis: we assume that each included study provides an estimate of the same population value. “Effect” is singular, because there is assumed to be one effect, which is estimated by each study.

-Random effects analysis: we assume that the included studies vary in what they estimate, and that we want a mean value. Here “Effects” is plural, because each study estimates a slightly different effect.